1848.] Construction of Iron Tension Bridges, 417 



12. Here, as regards the second defect, it may be objected, that 

 Mr. Dredge never intended his bridges to be sustained by tension in 

 the longitudinal beams at any point of their length, assuming in his 

 theory that "the tension at the centre is a cypher." The capacity of 

 the platform to resist compression in the two half curves, and not the 

 power against tension, being brought into action. 



13. Such has been Mr. Dredge's view and his rule of construction, 

 but experience on a full sized scale, (independent of the failure of the 

 bridges above noticed) has satisfied me that there is not strength in the 

 combination of the platform to resist compressive power. The defect 

 was proved as follows : — 



14. The whole of the iron work of a complete half curve of a 

 bridge of 120 feet span and 16 feet width of platform, was put up in 

 the Government Iron bridge yard on standards erected of masonry for 

 the purpose, thus : (See Fig. 1 .) 



The centre link was carried out horizontally in its proper position, 

 and attached to a wooden beam abutting against two trees. The cen- 

 tral ends of the longitudinal beams were left free, as shown above, the 

 other ends being built firmly into the masonry in their cast iron boxes, 

 whilst the half platform rested on three posts on each side, to preserve 

 the horizontality till the whole was put up. Every thing being in 

 position, the transverse beams, railing, &c. fixed, it is evident that on 

 the removal of the posts the structure would not fail, if there was 

 sufficient stiffness in the combination of the framing, to resist the 

 compressive action by the combined oblique pull of the auxiliary rods 

 depending from the chain; accordingly the posts were one by one 

 removed, when it was immediately seen that there was not that degree 

 of stiffness in the framing to resist the amount of compression from the 

 centre towards the standards, for when all the posts were removed, 

 about one-third of the length of the platform from the standards was 

 bowed out 25 inches, as in the annexed figure. (See Fig. 2.) 



There was at this time no extra load on the platform, and the con- 

 clusion seems obvious, that unless the longitudinal beams be kept 

 straight by tension from the opposite half curve, the framing could 

 hardly bear its own weight, far less be equal to a traffic load of 1 12ibs. 

 per square foot. In other words, the combination and scantling assigned 

 by Mr. Dredge have not strength to resist the compression ; the stability 



